In Linux, my preferred browsers are Opera 7 then Konqueror (Unless I'm SSH'ing, in which case there is only Lynx

)
In Windows... mmm. Opera 6, then Firefox, then IE

I love Opera

Fast, lightweight, good rendering and can switch compatibility modes, can save my 200-window session and let me return to it later... oh, and I'm addicted to mouse gesturing! Ahhh...
Downsides are that it is getting a bit bloaty - they keep adding crap to it that, frankly, I'm never gonna use (It has integrated e-mail, ICQ, news... I wouldn't be surprised if there was a Kitchen Sink menu item soon!

), and it costs money.
Firefox on it's own sucks arse; I don't know why everyone says it's lightweight and streamline because it uses almost as many resources as IE!! It's not faster than IE, but it is more responsive.
Firefox really starts to rule when you start hooking in XPI extensions 'tho - I just love how you can customise it up the wazoo

Whenever I see any knowledgeable techie using vanilla Firefox on it's own I have to resist hitting them over the head with a chair...